The present invention relates to connectivity between land-based cellular communications systems and user equipment located in airborne craft, and more particularly to technology that maintains wireless links between airborne equipment and ground-based equipment.
The world is becoming more and more connected, and this has led consumers to have increasing expectations of being able to be online and experience at least moderate data rates regardless of time and location. As one response to these expectations, the next generation of mobile technology, the so-called IMT-2020 (5G), targets high-speed mobility as one objective. The exemplary scenarios studied are high-speed trains and vehicles on freeways, but following the recent trend, it is expected that terrestrial in-flight broadband service for airplanes will be in the scope—either as direct communication between the User Equipment (UE) and base station, or via an access point (AP) onboard the aircraft which aggregates the traffic of some number of UEs and maintains a link to the base station.
In 2013 the Federal Communications Commission (FCC) took steps towards enabling better connectivity by assigning a 500 MHz wide subband in the 14 GHz radiofrequency (RF) band for in-flight air-to-ground broadband connection. The FCC's expectation is that by year 2021 there will be a demand for 15000 flights offering high-speed broadband connectivity to its passengers. By comparison, the availability in year 2013 was 3000 airplanes world-wide, and this was with connections that were deemed too slow and by far too expensive by consumers. The industry has noted that today's airline passengers expect the same level of broadband service that is available on the ground.
Several trials have been carried out offering terrestrial network coverage in lower frequency bands typically used for regular cellular networks. Recent advances on the regulatory side of aviation will, if properly exploited, greatly enhance and simplify in-flight broadband services that are based on terrestrial networks.
The high-level principles for maintaining coverage for mobile communication equipment on the ground are well known. So-called radio base stations are deployed at various geographical positions, and for a given mobile communication equipment, a “best-suited” base station is selected as the point of connection into the communications system. As the mobile communication equipment changes its position, the quality of its radio connection with the serving base station may deteriorate to the extent that a reselection is made, whereby a better-suited base station takes over as the serving base station.
Beam-forming technology can be used to facilitate radio communications between the airborne and ground-based equipment. But in conventional technology, the beam-forming equipment onboard the aircraft does not know the precise location of the terrestrial node it is transmitting to, nor does it rely on the aircraft's navigational information (e.g., orientation, position, velocity). As a result, the beams intended to reach the ground equipment need to be wide enough to ensure that at least some of the energy from the transmission actually reaches the terrestrial node.
But the use of wide beams for uplink transmissions (the direction from air to ground, in some earlier documents also known as the reverse link) may cause interference for terrestrial UEs in a wide area when terrestrial operator frequencies are reused for the air-to-ground (A2G) backhaul link. The interference reduces the uplink performance for terrestrial UEs in the area that is covered by the beam(s) because the signals scatter and therefore cannot be blocked by spatial filtering.
Moreover, when wide beams are used for uplink transmission, each backhaul link potentially gets degraded by time dispersion, and also in some cases frequency dispersion (depending on solution/technology being applied), due to scattering of the signal to be received.
Additionally, the scattering reduces the possibility of reusing physical resources in the uplink in different beams since signals from both beams will impinge from various directions, rendering it challenging in many situations to block the interference by spatial filtering.
In situations in which the same set of beams serves two or more aircraft, or when certain physical resources (PRBs) are reserved for terrestrial UEs, it is imperative that orthogonality be maintained between the signals transmitted for or by different backhaul users (aircraft). Otherwise, inter-carrier interference will cause a degradation in performance of the respective links.
Hence there is a continued need for a method by which interference due to scattering and inter-carrier interference can be minimized and capacity can be maximized when wireless links between airborne and ground-based equipment are being used.